Delivery of therapeutic agents has been a major problem. Oral administration is one of the most common and preferred routes of delivery due to ease of administration, patient compliance, and decreased cost. However, the disadvantages of this route include low or variable potency and inefficient adsorption of the therapeutic. This is particularly evident when the compound to be delivered is unstable under conditions encountered in the gastrointestinal tract. A variety of coatings and encapsulation methods have been developed in the art, but only a few are effective in addressing this issue. Still, there are therapeutic compounds that tend to be less active in the conditions of the gastrointestinal tract and must be administered in higher dosages to be adsorbed into the bloodstream in an effective amount.
A broad range of drug formulation systems have been developed to address the problem of optimal drug delivery and are based on incorporation of drug into a matrix that acts as a carrier. Factors considered in drug formulation include requirements that the system be non-toxic, non-reactive with the drug to be delivered, economical to manufacture, formed of readily available components, and consistent with respect to final composition and physical characteristics, including stability and release rate. It is also preferable that the drug delivery system is formed of materials easily removed from the body by normal physiologic processes.
Microparticle drug formulations can be used in numerous routes of administration, but are particularly well suited to pulmonary delivery. Advantages of the lungs for delivery of agents having systemic effects include the large amount of surface area and ease of uptake by the mucosal surface. U.S. Pat. No. 6,071,497, herein incorporated by reference, describes a pulmonary drug delivery system based on the formation of diketopiperazine microparticles as well as polymer-based microparticles.